We will study events in polyoma virus lytically infected and transformed cells that are determined by expression of the hr-t gene. This gene, encoding both the small and middle size tumor antigens, is responsible for most, if not all, of the changes in cell behavior that accompany neoplastic transformation. The role of the middle T antigen (mT) as both a substrate for cellular protein kinases, and a regulator of protein kinases, will be studied. Phosphoprotein metabolism will be studied in vivo with cells infected by various virus mutants specifically altered in mT as well as by wild type virus. Plasma membranes, isolated from these cells and containing either functional (wild type) or nonfunctional (mutant) middle T proteins will be incubated in vitro with Gamma-32P-ATP, and patterns of phosphorylation determined by two dimensional gel analysis of total membrane proteins. We will also utilize conditionally transformed rat cells expressing mT under control of the dexamethasone-regulated promoter of mouse mammary tumor virus. These cells will be titrated to express partial or complete transformation and used to look for potential targets for protein kinase activities triggered by middle T expression. Phosphatidyl inositol (PI) metabolism will be studied to determine if PI turnover rate is affected by the presence of mT antigen in vivo. In vitro PI kinase experiments will be done using immune precipitates containing wild type or mutant mT antigens. The hr-t gene also plays an important role in the virus growth cycle, at the level of capsid assembly. We will seek further evidence that assembly of the allpentamer capsid requires modification (phosphorylation and acetylation) of the major capsid, VP1, and that these modifications are regulated indirectly by the hr-t gene.